Cellulosic Fiber Containing Hydroentangled Nonwoven and Method for Producing It

ABSTRACT

The invention is directed to a low-dusting, soft hydroentangled nonwoven with improved strength, which comprises cellulosic fiber with a pre-determined average length as well as staple fibers, which are either synthetic fibers and/or modified cellulose fibers, as well as a method for making this nonwoven.

FIELD OF THE INVENTION

The present invention relates to a soft, absorbing, structurally strong and low-dusting nonwoven product, which can be used for example in hygiene products in a liquid transport or absorption layer as well as generally as a disposable nonwoven product. The product according to the invention is especially suitable for use for wiping both in dry and wet wipe applications. More specifically defined, the invention concerns a hydroentangled nonwoven, which contains cellulosic fiber and staple fibers, which are either synthetic fibers and/or modified cellulose fibers, and a method for producing this nonwoven.

STATE OF THE ART

In the field of use of nonwovens according to the invention, the product is required to possess, in addition to absorbance, softness, structural strength and it has to be dust-free. Structural strength and low dusting of the product are important properties also from the point of view of further processing of the fabric. In addition, in some applications the product has to be flushable into the sewer or the product is expected to be biodegradable. The economy of both the manufacturing method and the fiber raw materials is emphasized in the present nonwoven markets.

Cellulose containing products are absorbing, wherefore they are well suited for the above-mentioned fields of use. Cellulosic fiber is biodegradable natural material. A factor limiting the use of cellulosic fiber for example in nonwoven applications is its poor structural strength and dusting, which gives rise to especially much problems in the nonwoven manufacturing method and in the further processing. The reduction in structural strength and dusting is a problem also in the final use of the nonwoven, because for example when used as a towel, a cellulosic-based towel substrate often leaves fibers or fiber bundles on the surface to be wiped. The strength, especially the wet strength, of cellulosic products, is poorer than for example that of products made from synthetic fibers. In addition, the surfaces of the cellulosic products often have a coarse and paperlike feel.

The dusting of nonwoven products has traditionally been reduced by increasing the amount of binder in the nonwoven or by adding to the fabric dusting reducing surface treatment agent in the various stages of the manufacturing method. The additives, however, make the product stiffer, the adsorption and/or absorption property of the product may be weakened, the material costs are increased and the additive compounds can react with the lotions used in the wet wipe applications and may cause for example skin irritation. It has also been suggested, such as in the publication EP 1 280 659, various multilayer structures in order to reduce dusting.

In the publication EP 0 492 554 a nonwoven structure has been suggested for the production of a cellulose product, with the aim of improving the method of manufacture and the product properties by using as one component of the nonwoven a layer of continuous filaments. The nonwoven according to the publication EP 0 633 958 consisting of for example cellulosic fibers and synthetic fibers is characterized by patterning of the material on an apertured pattern unit, which reduces dusting of the product. In the invention according to the publication the proportion of cellulosic fiber is at the most 50 percent by weight.

According to the publication EP 0 833 977 it is suggested to improve the strength of a cellulose product by corona or plasma treatment. Various composite structures commonly used in nonwovens also provide strength to cellulosic fiber applications.

According to the above, efforts have been aimed at reducing dusting and improving the strength of nonwoven products containing cellulosic fiber by using binders, different layered structures, post-treatment of the fabric or modification of the surface structure. These treatments can lead to a weakening of other important product characteristics, for example to an increase of the coarseness of the surface and often require additional steps in the manufacturing process as well as the use of additives that are undesired in e.g. hygiene and towel applications.

Due to the problems appearing in the state of the art, there is a need to provide a hydroentangled nonwoven from cellulose and staple fibers, wherein synthetic and/or modified cellulose fibers are used as staple fibers. In addition, the product should be soft, structurally strong and low-dusting. According to the invention these favourable characteristics in the nonwoven are achieved by using cellulosic fiber with a specific fiber length distribution. The cellulosic fiber to be used according to the invention has a length weighted average length of less than 2.30 mm. As the improved product characteristics are affected already by the choice of the cellulose raw material, the properties obtained appear throughout the whole product and not only e.g. on the product surface as a result of subsequent surface treatments. No binders or separate support layers are needed.

SUMMARY OF THE INVENTION

The aim of the present invention is to obtain nonwoven product from cellulosic fiber (cellulose component) and staple fiber (synthetic and/or modified cellulose component) which is soft, has improved strength and is low-dusting.

It was observed that by using cellulose made from softwood grown in the north, Scandinavian cellulose or the like, a cellulose based nonwoven product is obtained having improved properties, which is structurally strong and less dusting. It was surprising that the desired properties could be obtained with cellulose having a low length weighted average length; less than 2.30 mm. In addition, in the cellulose according to the invention, the percentage proportion of shorter fibers is greater than in the celluloses commonly used in commercial nonwovens.

In the nonwoven according to the invention, the proportion of cellulose is 45% by weight or more, preferably 50% by weight or more, and especially advantageously over 50% by weight, the remainder being synthetic fibers and/or modified cellulose fiber, preferably modified cellulose fiber. Increasing the proportion of cellulosic fiber improves the absorbance and the ecology of the product and decreases raw material costs. Factors limiting the amount of cellulose are the decrease of the structural strength, increase of dusting, manufacturability of the product and its further processing. The maximum amount of cellulose in the product is also partly determined by the intended use.

The method according to the invention for the continuous production of a cellulose containing nonwoven comprises the steps of forming a web from the fiber components (cellulose and synthetic and/or modified cellulose fiber) by dry laying, combining the fiber webs if necessary and bonding at a first hydroentanglement station, which can be followed by one or more hydroentanglement stages. After entanglement, water is removed from the fabric, which is subsequently dried and winded to a roll.

DETAILED DESCRIPTION OF THE INVENTION Brief description of the drawing

FIG. 1. A schematic drawing of a hydroentanglement line suitable for the method according to the invention.

The term “average fiber length” means the length weighted average length of the cellulosic fiber obtained as a result of measurement carried out on a Kajaani FS-200 device.

The term “softness” means the mental image perceived of the softness of the fabric by means of the tactile sense when feeling the fabric with the fingers, as compared to other products.

The term “low-dusting” refers to the structural strength of the nonwoven, that is the improved entanglement of the fibers as a result of the bonding process. This is established i.a. as the nonwoven durability when subjecting the fabric to abrasion of the Atlas Pilling—test type (more detailed description in connection with the test methods). In the Atlas Pilling—test the number of particles released from the fabric during testing is measured. The test illustrates well the behaviour of the nonwoven fabric in a wiping situation, where the fabric is subjected also to a forceful abrasion. In addition, the test makes it possible to anticipate the behaviour of the nonwoven during the further treatment of the fabric, where the fabric is subjected to mechanical stress, such as abrasion.

The invention is described in the following with reference to the appended drawings and tables.

The product according to the invention is manufactured for example on the hydroentanglement line shown in FIG. 1. In this method, both components of the fiber mixture are separately formed into a web, whereby a cellulose component and a synthetic and/or a modified cellulose component is obtained. The nonwoven according to the invention is formed when combining the fiber webs at a first hydroentanglement station (9). The first component of the fiber mixture, which is formed from a synthetic fiber, such as polyester, or advantageously from a modified cellulose fiber, such as viscose, is fed from a feeding unit (1) to a carding unit (3) and is formed into a fiber web (4) by carding. Instead of a carding unit (3) also e.g. an air carding unit can be used. The carded fiber web (4) is transported with a wire to the first hydroentanglement station (9) to be combined. The second component of the nonwoven according to the invention, the cellulose component, is brought from a hammer mill (5) to a former (7) wherein a cellulose web (8) is formed by dry-laying. Also the cellulose web is transported on its own wire to the first hydroentanglement station (9) to be combined with the second fiber web component of the fabric. The cellulose web is applied onto the first fiber web from the carding unit and the first entanglement is carried out on the cellulose fiber side of the combined web.

The nonwoven can comprise more than two web layers, for example so that the cellulose fiber is formed into a web between two synthetic and/or modified cellulose fiber webs. It is also possible to form the web from a mixture, which comprises both cellulose fiber and synthetic and/or modified cellulose fiber at a suitable ratio.

According to an embodiment, the nonwoven is turned over and transported to the next hydroentanglement station (11). The second bonding thus takes place from the synthetic and/or modified cellulose fiber side. According to a further embodiment the nonwoven is transported still to a third hydroentanglement station (13), where the entanglement again takes place from the cellulose side of the nonwoven. According to an embodiment, smooth wires are used in all hydroentanglement stations, for example 75 mesh wires.

The visual properties of the nonwoven can be improved by patterning, that is by changing the means for supporting the web in at least one hydroentanglement unit, advantageously in the last hydroentanglement unit (13) to an apparatus for patterning the nonwoven, as has been described for example in the patent application FI 20040848. Advantageously patterning of the nonwoven takes place by supporting the fiber web or nonwoven from the synthetic and/or modified cellulose side on the surface of the patterning means.

After entanglement, the nonwoven is transported through a foulard device (15) where water is removed. Thereafter the nonwoven is guided through a drying oven (17) and wound to a roll on a winder (19).

According to the invention cellulose having an average fiber length of less than 2.30 mm, is used in the nonwoven manufacturing process. Such cellulose is obtainable from softwood grown in the north, cellulose fiber made from such wood having a length weighted average length, which varies between 1.90 and 2.30 mm. The percentage proportion of short fibers in the cellulose used in the method and product according to the invention is thus higher than in comparison celluloses. This cellulose mass is also characterized by a high number of fibers per cellulose gram, which gives a more covering product and makes it possible to manufacture a fabric having a lighter grammage. In the following table 1, a more precise fiber length distribution for two comparison celluloses as well as for a cellulose (cellulose A) used in the product and the method according to the invention having a specific fiber length distribution. For the cellulose A used in the examples according to the invention described below the manufacturer informs that the average length of the said fiber remains between the limits 2.10 mm and 2.30 mm.

TABLE 1 Fiber Fiber distribution Fiber distribution average % by weight Fiber Mill/g length 0-1.0 1.0-2.0 2.0-7.2 number 0-1.0 1.0-2.0 2.0-7.2 Sample mm mm mm mm Mill./g mm mm mm Comp. 2.59 13.9 22.4 63.8 4.1 0.57 0.92 2.6 cellulose 1 Comp. 2.53 13.5 23.4 63.1 4.1 0.55 0.96 2.6 cellulose 2 Cellulose 2.17 17.9 26.8 55.3 5.7 1.02 1.5 3.2 A

As a raw material, cellulose fiber is economically advantageous and its use in nonwoven applications is warranted also by the other facts mentioned earlier in the application. However, the relative proportion of cellulose in nonwovens where staple fibers are used, has traditionally remained low. According to this invention, when using cellulose with a specific fiber length distribution, it is possible to achieve also in a staple fiber product a high percentage proportion of cellulose, without losing desired properties for the product, such as softness. According to the invention, it is possible to use cellulosic fiber having a specific fiber length distribution in an amount of over 50% by weight and obtain a low-dusting product without adding binder or applying separate post-treatments. The proportions of the different components are determined as the weight proportions of the fiber webs prior to the entanglement stage. Increasing the cellulose proportion in the fiber mixture decreases the material costs. Synthetic fibers and/or modified cellulose fibers, advantageously only modified cellulose fibers, can be used as the second component in the fiber mixture.

The synthetic fiber can be any synthetic fiber suitable for the application, for example polyester or polypropylene, advantageously, however, polyester. The modified cellulose fiber is advantageously viscose. It is also possible to use a mixture of a synthetic fiber and a modified cellulose fiber in the product according to the invention. The synthetic fiber and/or the modified cellulose fiber is a staple fiber according to the invention. The fineness of the synthetic fiber and/or the modified cellulose fiber in the product according to the invention is between 1.1 dtex and 3 dtex, advantageously the fineness of the synthetic fiber or the modified cellulose fiber is 1.7 dtex. The length of the synthetic or the modified cellulose fiber to be used in the product according to the invention is between 5 mm and 60 mm, advantageously the length of the fiber to be used is between 30 mm and 60 mm, especially advantageously the length of the synthetic fiber or the modified cellulose fiber is between 38 mm and 40 mm.

The nonwoven according to the invention consists of the above-described cellulosic fiber and synthetic and/or modified cellulose fibers. In the product according to the invention staple fibers are used. Advantageously the nonwoven according to the invention consists of two separately dry-laid fiber components; the cellulose component and the synthetic and/or modified cellulose fiber component. The synthetic fiber to be used in the nonwoven according to the invention is advantageously a polypropylene fiber, especially advantageously polyester. The modified cellulose fiber to be used in the nonwoven according to the invention is preferably a viscose fiber. An especially preferred embodiment of the nonwoven according to the invention is comprised of two separately dry-laid fiber components, which are a cellulose component and a viscose component.

In the nonwoven according to the invention the proportion of cellulose is 45% by weight or more, advantageously 50% by weight or more and especially advantageously over 50% by weight, the remainder being synthetic fiber and/or modified cellulose fiber, preferably modified cellulose fiber. The proportions of the different components are determined as weight proportions of the fiber webs before the entanglement stage. According to the invention it is possible to combine, by using cellulose with a specific average length, the good properties which are characteristic to cellulose fiber with the other important properties, which are required of the end product, such as structural strength. The mixing ratio is determined by limits set by the manufacturability, further processing and intended use of the product.

In the following the testing methods are described which have been used for determining the fiber length of the cellulose and for testing the nonwovens manufactured in the examples.

Determination of the Fiber Length Distribution

The average length of the cellulose fibers and the fiber length distribution was determined optically on a Kajaani FS-200 fiber length analyzer.

Atlas Pilling-Test

By means of the Atlas Pilling-test one can determine the amount of cellulose released from a nonwoven product when the product is subjected to wear. The method is based on the SFS 3378 standard. A rotor rotating at a high speed throws free test specimens of predetermined size in a cork lined cylinder so that they continuously wear against the cork, each other and the rotor.

In the test method four test pieces are taken (125 mm*125 mm) from the whole width of the specimen at a distance of at least 100 mm from the borders. The specimen is conditioned for at least 12 hours under standard conditions. Prior to each test the testing cylinder is cleaned and a cork plate is attached to the inner wall of the cylinder so that an unused side is against the specimens. All three 20 minutes test runs are performed on the same surface of the cork plate. After each 20 minutes run the test specimens are individually cleaned by brushing with a brush and suctioning off the free dust into a suction device. After weighing the change in weight is calculated as percentage of the original weight.

The softness of the Nonwoven

The softness of the nonwoven was determined with a blind test in which six different persons estimated the softness of four nonwoven samples. In the test two nonwoven products according to the invention with different mixing ratios were used. As a comparison sample two nonwoven fabrics were used, wherein as the cellulose a comparison cellulose was used with different mixing ratios. The softness of the nonwovens was estimated on a five-step scale. The value 5 was assigned to a nonwoven, which was evaluated as very soft, and the value 1 was assigned to a nonwoven, which was evaluated as very coarse.

The Strength of the Nonwoven

The tensile strength of the fabric was determined with a Zwick Z2.5—tensile testing machine. The Edana ERT 20.2-89 method was applied to determine the strength. In the testing method, 8 test specimens are cut from a nonwoven in the machine and the cross direction. The size of the test specimen is 50 mm*250 mm. A pair of tensile test specimens is attached to the jaws of the tensile testing machine, whereafter the drawing is carried out. The results are given both in the machine (MD) and the cross direction (CD).

The Wet Strength of the Nonwoven

The wet strength of the nonwoven was measured according to the Edana ERT 20.2-89 method. In the method the specimen is soaked in water, to which 1 g/l of Triton X-100 for decreasing the surface tension has been added. 6 specimens are cut from the nonwoven in the machine (MD) and cross direction (CD). The size of the specimen is 50 mm*250 mm. The test specimens are soaked for 1 h. The superfluous water is pressed out from the test specimens by hand and a pair of tensile test specimens is attached to the jaws of the tensile testing machine. A Zwick Z2.5-tensile testing machine is used for drawing. The results are given both in the machine (MD) and the cross direction (CD).

The Thickness and Grammage of the Nonwoven

The Edana ERT method 30.5-99 was used for determining the thickness of the fabric. The Edana ERT method 40.3-90 was used for determining the grammage.

In the following the invention is illustrated in more detail by means of the following examples. The examples do not limit the scope of the invention, which is defined by the claims.

EXAMPLE 1

A nonwoven product and a comparison product were made on a nonwoven production line as disclosed in the FIG. 1. Both products were manufactured in the same production run by changing the cellulose fiber from the comparison cellulose to a cellulose having a predetermined fiber length distribution in accordance with the invention, without stopping the line. In the first hydroentanglement on the nonwoven production line, entanglement pressures of 40, 52 and 70 bar were used. In the following entanglement station entanglement pressures of 50 and 50 bar were used. Still a third entanglement was carried out on the fabric where the entanglement pressure was 68 and 64 bar. In all three hydroentanglement units a wire was used having a smoothness of 75 mesh.

In the nonwoven product according to the invention cellulose A disclosed in the table 1 was used. As the second fiber component a viscose fiber was used having a fineness of 1.7 dtex and fiber length of 40 mm. The composition of the product according to the invention was 50% by weight cellulose and 50% by weight viscose. The grammage of the nonwoven according to the invention was 55 g/m². In addition, a comparison nonwoven was made using the comparison cellulose according to the table 1. The composition of the comparison nonwoven was 50% by weight cellulose and 50% by weight viscose. The grammage of the comparison nonwoven was also 55 g/m².

TABLE 2 Property Product according Comparison Unit to the invention product Grammage g/m² 56.8 57.0 Thickness mm 0.35 0.37 Strength MD N/5 cm 86.6 76.47 CD N/5 cm 20.7 17.02 Wet strength MD N/5 cm 34.5 33.71 CD N/5 cm 11.2 8.8 Atlas Pilling 1 % −5.5 −11.1 2 % −9.9 −19.3 3 % −14.8 −25.1

Surprisingly it was observed that by using cellulose having a predetermined fiber length distribution in accordance with the invention clearly less dusting of the nonwoven product is obtained, which is evident from the Atlas Pilling test results. The nonwoven remains intact and the amount of particles released therefrom is small as compared to the state of the art. In addition it was observed that both the dry and wet strength of the product was improved.

EXAMPLE 2

A nonwoven product according to the invention was made on a nonwoven production line according to the FIG. 1. The composition of the product was 55% by weight cellulose (cellulose A) and 45% by weight viscose. The production speed of the nonwoven production line was 1.33 times that used in the examples 1 and 3. In the first hydroentanglement on the nonwoven production line entanglement pressures of 39, 60 and 76 bar were used. In the following hydroentanglement station entanglement pressures of 38 and 40 bar was used. The fabric was subjected to still a third entanglement where the entanglement pressures were 66 and 70 bar. In all three hydroentanglement units a wire was used having a smoothness of 75 mesh.

TABLE 3 Property Product according Unit to the invention Grammage g/m² 62.2 Thickness mm 0.38 Strength MD N/5 cm 76.8 CD N/5 cm 19.3 Wet strength MD N/5 cm 32.1 CD N/5 cm 9.0 Atlas Pilling 1 % −5.0 2 % −10.7 3 % −17.7

The use in accordance with the invention of a cellulose having a predetermined fiber length distribution at a proportion of 55% by weight results in a clear reduction of the dusting of the nonwoven product, which is evident from the Atlas Pilling test results. The fabric remains intact and the amount of particles released therefrom is small as compared to the state of the art. The strengths in the nonwoven production runs of example 2 decreased due to the increase of the production speed, which is seen especially in the MD dry strength.

The Softness of the Nonwoven

The softness of the nonwoven from two products according to the invention and two comparison products were tested in accordance with the method described above. The test results are given in the table 4. The cellulose A means the cellulose fibers having a pre-determined average length as disclosed in the table 1.

TABLE 4 Comparison Comparison Cellulose A Cellulose A cellulose cellulose CEL/VIS CEL/VIS CEL/VIS CEL/VIS 55/45 50/50 50/50 65/35 55 gsm 55 gsm 55 gsm 80 gsm Very soft [5] IIII I I 0 Soft [4] II III I 0 Not soft, not 0 0 II 0 coarse [3] Coarse [2] 0 I II II Very coarse [1] 0 I 0 IIII Average 4.7 3.3 3.2 1.3 Median 5 4 3 1 Average 0.52 1.5 1.2 0.52 scattering N 6 6 6 6

Based on the test results the nonwoven according to the invention was clearly softer than the comparison product. The softness of the nonwoven according to the invention increased and softness of the comparison product decreased when the proportion of the cellulose increased in the product. The product according to the invention wherein 55% by weight of cellulose having a pre-determined average fiber length was used in accordance with the invention, was considered to be especially soft.

EXAMPLE 3

A nonwoven product according to the invention and a comparison product were made on a nonwoven production line according to the FIG. 1. In the first hydroentanglement on the nonwoven production line entanglement pressures of 40, 67 and 70 bar were used. In the following hydroentanglement station an entanglement pressure of 40 bar was used. The fabric was subjected to still a third entanglement where the entanglement pressure was 80 bar. In all three hydroentanglement units a wire was used having a smoothness of 75 mesh.

In the nonwoven product according to the invention cellulose A according to the table 1 was used. As the second fiber component a polyester fiber having a fineness of 1.7 dtex and fiber length of 38 mm was used. The composition of the product according to the invention in the web forming stage was 50% by weight of cellulose and 50% by weight polyester. The grammage of the nonwoven according to the invention was 70 g/m². In addition a comparison fabric was made using the comparison cellulose 2 according to the table 1 using the same production parameters and using the same polyester fiber as the second fiber component. The composition of the comparison nonwoven in the web formation stage was 50% by weight cellulose and 50% by weight polyester. The grammage of the comparison fabric was 68 g/m².

TABLE 5 Property Product according Comparison Unit to the invention product Composition⁽* Cellulose % 49.1 46.9 PES % 50.9 53.1 Grammage g/m² 70.3 67.6 Thickness mm 0.47 0.48 Strength MD N/5 cm 169 167 CD N/5 cm 43 36.4 Wet strength MD N/5 cm 115 121 CD N/5 cm 31.1 22.8 Atlas Pilling 1 % −3.8 −3.7 2 % −5.5 −6.6 3 % −7.2 −9.0 ⁽*The compositions of the product in table 5 are based on laboratory measurements carried out on the final product.

It was observed that by using the cellulose according to the invention dusting of the product can be reduced which is evident from the Atlas Pilling test results. Using the cellulose according to the invention resulted also in an increase in the product strength. The poorer wet strength in the MD direction is due to the larger proportion of polyester in the comparison product. 

1. Low-dusting, soft hydroentangled nonwoven with improved strength, which comprises cellulosic fiber and as staple fiber synthetic and/or modified cellulose fiber, wherein the average length of the cellulosic fiber is less than 2.30 mm.
 2. The nonwoven according to claim 1, wherein the average length of the cellulosic fiber is between 1.90 mm and 2.30 mm.
 3. The nonwoven according to claim 1, wherein the average length of the cellulosic fiber is between 2.10 mm and 2.30 mm.
 4. The nonwoven according to the claim 1, comprising at least one layer of cellulosic fiber and at least one layer of staple fiber.
 5. The nonwoven according to claim 1, wherein the staple fiber is viscose.
 6. The nonwoven according to claim 1, wherein the staple fiber is polyester.
 7. The nonwoven according to claim 1, wherein the cellulose fiber is made from northern softwood.
 8. The nonwoven according to claim 1, wherein the proportion of cellulosic fiber in the fabric is 45% by weight or more.
 9. The nonwoven according to claim 1, wherein the proportion of cellulosic fiber in the fabric is 50% by weight or more.
 10. The nonwoven according to claim 1, wherein the proportion of cellulosic fiber in the fabric is over 50% by weight.
 11. A method for the production of a low-dusting, soft hydroentangled nonwoven with improved strength, which nonwoven comprises cellulosic fiber and as staple fiber synthetic and/or modified cellulose fiber and which method comprises the steps of: forming the web from cellulosic fiber and staple fiber by dry-laying, bonding the web by hydroentanglement, removing water from the nonwoven, and drying the nonwoven and winding to form a roll, wherein the cellulosic fiber web is comprised of cellulosic fibers having an average length of 2.30 mm or less.
 12. The method according to claim 11, wherein the average length of the cellulosic fiber is between 1.90 mm and 2.30 mm.
 13. The method according to claim 11, wherein the average length of the cellulosic fiber is between 2.10 mm and 2.30 mm.
 14. The method according to claim 11, wherein at least one web is formed from cellulosic fiber and at least one web from staple fibers. 